Leak detection with thermal imaging

ABSTRACT

An infrared camera is used to detect leakages in a heat exchanger by an imaging process that indicates temperature differences between a heat exchanger and a pressurized gas therein. The temperature differences are created by cooling or heating either the pressurized gas or the heat exchanger, and any leakages are visually observable by the resultant image which is representative of the temperature differences. The process can be accomplished with the use of ambient air rather than the commonly used trace gases which are less desirable because of economical and environmental reasons.

BACKGROUND OF THE INVENTION

This invention relates generally to heat exchangers for air conditioningsystems and, more particularly, to a method and apparatus for leaktesting of heat exchangers to be charged with refrigerant.

There are many different ways of testing the integrity of a coil or heatexchanger used in residential, commercial or industrial air conditioningand heating systems, but all of them require the use of a trace gas anda surrounding matrix against which escape of the trace gas is detectableeither visibly or by the use of instruments. Such trace gases, rangingfrom refrigerants to inert gases, are used either to pressurize thecomponent prior to leak detection of the outer geometry of the part orforming a surrounding environment on the component itself while thelatter is subject to internal vacuum.

The cost of the testing process is dictated not only on the originalequipment initial costs, but on the cost of use of the trace gas initself. The very nature of this approach imposes a potential impact onthe factory environment due to the escape of gas and non-containedleakage points.

Trace gas escapes to the factory environment are greatly undesirable notonly because they represent waste in terms of cost (non recyclable norreusable gas emissions), but also because of the increasingenvironmental codes and regulation in all the major HVAC regions of theworld including North, Central and South America, and particularly inAsia and Europe.

Another variation of the trace gas systems is the so called “Air underWater” process, very common in today's HVAC industry because of itsrelative simplicity. This process uses the principle of pressurizing theheat exchanger or coil with air to a specific design pressure, cappingthe heat exchanger so that it will maintain the pressure, and thensubmerging the component in clear water to detect leaking hole size andlocation by visual inspection of the air bubbles. This process has thedisadvantage of also producing water emissions. As of now, there arevery few testing systems that are “clean” and emission free.

Thermal imaging systems work on the principle that all bodies have agiven amount of radiation of heat depending on their actual surfacetemperature, surrounding energy sources (i.e. light, heat, etc. . . ),surface conditions and physical properties of the material that they aremade of. A special infra-red camera device adjusted to work on theinfra-red light spectrum frequency range is able to detect the differenttemperature gradient areas or zones on the body surface. This videoimage is then fed to a computer for imaging processing, so that it cangraphically display the temperature distribution on a screen foranalysis and interpretation. Thermal imaging systems are commerciallyavailable with different detection sensitivities for use in diverse waysin the medical and industrial fields, such as insulated steam pipeleak/breakage point location for maintenance work, main waterunderground pipe leaks, medical body scans, etc. . .

It is therefore an object of the present invention to provide animproved method and apparatus for leak testing of heat exchanger coils.

Another object of the present invention is the provision for a leaktesting method which reduces the occurrence of introducing trace gasesto the environment.

Yet another object of the present invention is the provision forreducing the waste that results from loss of trace gases during leaktesting.

Still another object of the present invention is the provision for aheat exchanger leak testing process that is efficient and effective inuse.

These objects and other features and advantages become more readilyapparent upon reference to the following description when taken inconjunction with the appended drawings.

SUMMARY OF THE INVENTION

Briefly, in accordance with one aspect of the invention, the heatexchanger to be tested is pressurized, and either the heat exchanger orthe pressurizing vapor is heated or cooled so as to create a significanttemperature gradient between the heat exchanger and the pressurizedvapor. An infrared camera is then used to create an image of the heatexchanger as representative of both the temperature of a heat exchangerand the temperature of any gas that is leaking therefrom. The images canthen be analyzed to locate and fix any leaks in the heat exchanger.

In accordance with another aspect of the invention, the heat exchangeris pressurized with ambient air, such that the cost of trace gases thatare otherwise used, are eliminated, as well as the need to flush theheat exchanger after testing.

By another aspect of the invention, the temperature gradient between theheat exchanger and the contained vapor is obtained by heating or coolingthe air that is pumped into the heat exchanger. The gradient may beincreased by oppositely cooling or heating the heat exchanger.

By yet another aspect of the invention, a black body is placed behindthe coil to prevent the reflection of light that would otherwiseintroduce errors into the imaging process.

By still another aspect of the invention, the infrared camera may beapplied to generate representative signals which are processed forgenerating input and feedback signals to peripheral equipment for thepurpose of improving quality control.

In the drawings as hereinafter described, a preferred embodiment isdepicted; however various other modifications and alternateconstructions can be made thereto without departing from the true spirtand scope of the invention.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic illustration of the leak testing apparatus inaccordance with the preferred embodiment of the invention.

FIG. 2 is a graphic illustration of the operational parameters forthermal image leak detection on heat exchangers.

FIG. 3 is a pictorial illustration of the stages of transition as theleak detection process occurs.

FIG. 4 is a process flow chart indicating the steps of the process inaccordance with the preferred embodiment of the invention.

DESCRIPTION OF THE PREFERRED EMBODIMENT

Referring now to FIG. 1, the invention is shown generally at 10 asapplied to a heat exchanger or coil 11 to be tested. The coil 11 ispressurized by a compressor or generator 12 to a relatively low pressuresuch as 5 psi, for example. A preferred gas medium is ambient air.

An infra-red camera 13 is set up in the vicinity of the coil 111 suchthat it can detect any leakage from the coil 11 by the temperaturedifference between the coil 11 and the escaping vapor. In order toexpose the various locations of possible leakage from the coil 11, itwill generally be necessary to move either the coil 11 or the camera 13.If the coil is moved, the camera 13 can remain stationary while aprogrammed fixture can be made to automatically move the coil 11 to thevarious positions that will enable the camera 13 to be aimed at thepossible areas of leakage. In the alternative, the coil 11 may remainstationary, with the camera 13 being moved in a programmed manner toeffectively sweep the heat exchanger surfaces to scan for possibleleaks.

After the infra-red camera 13 has sensed the different temperaturegradient areas or zones on the heat exchanger 11 caused by of theleakage of the hot air in the vicinity of the cooler coil surfaces, theresultant signals are sent to an image processor 14 and then to acomputer 16 for the generation of a graphic display of the temperaturedistribution. Such a graphic display as shown in FIG. 3 can then beanalyzed and interpreted in a manner to be described more fullyhereinafter.

Keep in mind that, while it is imperative with the present system thatthere be a temperature difference between the leaking gas and thesurrounding surface of the coil, those relative temperatures can beaccommodated in any of a number ways. One approach is to simplyintroduce the hot air into the relatively cool coil and to allow thecoil to be slowly heat up by the hot air in a transient manner. Withthis approach, it is necessary to calibrate the system to recognize therelative temperature relationship with changes of time. That is, thecoil 11 will tend to be heated up by the hot air, and the hot air willtend to be cooled down by the coil 11. Thus, when the temperaturedifference between the two is finally recorded by the camera 13, thedegree of heating/cooling of the two mediums must be taken intoconsideration for the proper display and analysis of the test results.

Another factor that must be considered in the calibration of the systemis the fact that there will be some cooling of the escaping leaked gassimply because of the expansion process that occurs with the leakagefrom the pressurized container.

The relative temperature relationship is shown in FIG. 2 wherein thetemperature of each of the coil 11 and that of the air is shown as afunction of time. There, it is will be seen that if time equals t₀, thetemperature differences are greatest, and at time t₃, they are equal andno meaningful data can be obtained. Further, at time equals t₂ thetemperature gradient is quite small and the representative imaging andanalysis would be difficult. Accordingly, it is desirable to obtain thedata as quickly as possible after t₀. Thus, a typical process scan wouldoccur, preferably, between t₁ and t₂ as shown. Again, the analysis ofthe resulting image of such transient operation will be more fullydescribed with reference to FIG. 3 hereinafter.

Rather then operating on a transient basis as described hereinabove, itis also possible to maintain the coil 11 at a constant ambienttemperature by the use of forced convective air or the like. While thisadds an extra step, as well as the need for additional equipment, itdoes simplify the analysis because of the coil 11 remaining at thetemperature t₁ throughout the test. Of course, there are still transientaspects to this approach since the hot air within the coil 11 will tendto be cooled by the substantially cooler surface of the coil 11.

Rather then using hot air and a cool coil, the arrangement may bereversed. That is, the heat exchanger or coil 11 may be heated to ahigher temperature, with the pressurizing air being at ambient or atsome other cooler temperature. In doing this, one approach would be tosimply heat up the heat exchanger first, and then inject the cooler gas,with each temperature being allowed to drift toward the other in atransient manner as discussed hereinabove. In the alternative,additional heat may be added to the heat exchanger in order to maintaina constant temperature, such that the only transient consideration thatneeds to be given is that of the warming of the cool air with time.

With any of the above approaches, it is important that the priortemperature component, whether it is the coil 11 or the air, remainsbelow the critical melting or middle metallargical transformation pointof the heat exchanger base metal.

A non-reflective, high absorbance black surface screen or booth 17 maybe placed around the coil to reduce error that may be caused byreflections from other surrounding structures.

Referring now to FIG. 3, there are shown four sequential views of thegraphic display resulting from a leak test in accordance with the abovementioned process. The respective times of the various images are shownin the lower left, whereas the temperature color scale is shown at theright. It will thus be seen that a complete sequence from frame 1 toframe 4 occurs in about seven seconds. During that time, the colordifference resulting from temperature gradient changes from a maximum inframe 1 to a minimum in frame 4. That is, in frame 1, the coil is brightcolored and the leakage location is dark colored. As the coil cools downand the air heats up, the color differences tend to fade as we proceedfrom frames 2-4. It therefore be seen that, with such a transientoperation, it is important to use the images and analyze the data duringthe early portions of the transient cycle.

Although the images are shown in black and white, the imaging equipmentthat is available for display of the data in the manner describedhereinabove is quite capable of displaying the test results in color,with cooler temperatures being shown in blue and the warmer temperaturesbeing shown in colors on the other side of the spectrum such as yellowor white. Such a colored display therefore provides more flexibility andcapability for analysis.

While some of the variables that must be considered in the calibrationof the present system were discussed hereinabove, there are othervariables that will affect the expected temperature curves of the fluidand the surface of the heat exchanger as follow:

-   -   1. Physical variables such as air flow, initial air temperature,        air humidity content (best if dried), initial coil surface        temperature, ambient air temperature, air pressure, leak size,        and number of leak points.    -   2. Coil surface radiation, absorption, and thermal conductive        properties.    -   3. Distance of coil to black body screen, as well as absorption        surface properties of the materials that the black body screen        is made of; and    -   4. Ability to physically isolate the test coil from light and        heat sources; preferable to place it within a booth.

All of the above parameters come into play in defining the actualoperational testing time interval. This interval is determined by fixingthe following key variables:

-   -   1. Humidity content of trace air (preferable zero percent)    -   2. Air flow (determined by combining internal pressurization of        coil which is a coil design known factor and the coil internal        tube inside diameter);    -   3. Ambient temperature and humidity content controlled within        the test booth; and    -   4. Scan path for the high arc camera (i.e., single pass, double        pass etc.).

5. After these key variables have been established, a series of testsare then run to adjust the remaining parameters depending on the coilsize. This would be done once per coil size only, and then the resultswould be stored in the control computer for further use during testing.

The time interval can be extended by maintaining the surface of themetal of the coil from achieving thermal equilibrium with the escaping,cooling air by providing some mechanism to prevent coil heating, such asthe invective air flow system suggested above or the use of an ambientair blanket on the coil on all its surfaces expect the interrogatedarea. The larger the process time interval, the closer will the scansystem to an on-line, real time imaging system that can provide a highdegree of operator interaction and ability to manual interrogate areasduring the test.

Referring now to FIG. 4, a typical flow chart is shown for the conductof a leak test of a HVC coil. While this process shows the specificsequence of general and specific steps to be taken, it will beunderstood that the sequence and the particular steps can besubstantially varied such as discussed hereinabove without departingfrom the true sprit of the present invention.

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 19. A method of detectingleaks in heat exchanger of the type having an internal conduit throughwhich refrigerant is intended to pass for purposes of heat transferbetween the refrigerant and a medium passing externally over said heatexchanger, comprising the steps of: pressurizing said heat exchangerwith a vapor which is at a different temperature from that of the heatexchanger; and exposing areas of the heat exchanger to a temperaturesensitive device so as to detect leakage of said vapor from the heatexchanger.
 20. A method as set forth in claim 19 wherein said vapor is anon-refrigerant.
 21. A method as set forth in claim 20 wherein saidvapor is ambient air.
 22. A method as set forth in claim 19 andincluding the step of heating said heat exchanger to increase thetemperature difference between said heat exchanger and vapor.
 23. Amethod as set forth in claim 19 and including the step of cooling theheat exchanger to increase the difference between the temperatures ofthe heat exchanger and the vapor.
 24. A method as set forth in claim 19and including the step of positioning a black body behind the heatexchanger to reduce the error that would otherwise occur by reflectionof light to a temperature sensitive device.
 25. A method as set forth inclaim 19 wherein said temperature sensitive device is an infraredcamera.
 26. A method as set forth in claim 25 and including the furthersteps of generating signals of the respective temperatures andresponsively constructing an image representative thereof.